CH as a Molecular Gas Tracer and C-Shock Tracer Across a Molecular Cloud Boundary in Taurus

Abstract

We present new observations of all three ground-state transitions of the methylidyne (CH) radical and all four ground-state transitions of the hydroxyl (OH) radical toward a sharp boundary region of the Taurus molecular cloud. These data were analyzed in conjunction with existing CO and dust images. The derived CH abundance is consistent with previous observations of translucent clouds ($0.8\le A_v\le 2.1$ mag). The $X(\mathrm{CH})$-factor is nearly a constant at $(1.0\pm 0.06)\times 10^{22}$ $\mathrm{c}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}\mathrm{k}{\mathrm{m}}^{-1}\mathrm{s}$ in this extinction range, with less dispersion than that of the more widely used molecular tracers CO and OH. CH turns out be a better tracer of total column density in such an intermediate extinction range than CO or OH. Compared with previous observations, CH is overabundant below 1 mag extinction. Such an overabundance of CH is consistent with the presence of a C-shock. CH has two kinematic components, one of which shifts from 5.3 to 6 km s${}^{-1}$, while the other stays at 6.8 km s${}^{-1}$ when moving from outside toward inside of the cloud. These velocity behaviors exactly match with previous OH observation. The shifting of the two kinematic components indicates colliding streams or gas flow at the boundary region, which could be the cause of the C-shock.

arXiv admin note: text overlap with arXiv:1601.03165

References in corpus (5)

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• The Transition from Diffuse Molecular Gas to Molecular Cloud Material in Taurus
• The Interstellar Medium: The Key Component in Galactic Evolution and Modern Cosmology
• Chemical variations across the TMC-1 boundary: molecular tracers from translucent phase to dense phase
• Precise Measurements of CH Maser Emission and Its Abundance in Translucent Clouds
• The evolution of CH in Planck Galactic Cold Clumps
• The Drag Instability in a 1D Isothermal C-Shock
• Physical Properties and HI-to-H2 Transition Across Taurus Linear Edge
• Revealing the drag instability in one-fluid nonideal MHD simulations of a 1D isothermal C-shock
• First Search for Pulsed CH Maser Emission Stimulated by a Pulsar